The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation

The Astrophysical Journal (Impact Factor: 6.73). 02/2011; DOI:10.1088/0004-637X/736/2/94
Source: arXiv

ABSTRACT We combine ground and space-based photometry of the Galactic globular cluster
47 Tuc to measure four independent lines of evidence for a helium gradient in
the cluster, whereby stars in the cluster outskirts would have a lower initial
helium abundance than stars in and near the cluster core. First and second, we
show that the red giant branch bump (RGBB) stars exhibit gradients in their
number counts and brightness. With increased separation from the cluster
center, they become more numerous relative to the other red giant (RG) stars.
They also become fainter. For our third and fourth lines of evidence, we show
that the horizontal branch (HB) of the cluster becomes both fainter and redder
for sightlines farther from the cluster center. These four results are
respectively detected at the 2.3$\sigma$, 3.6$\sigma$, 7.7$\sigma$ and
4.1$\sigma$ levels. Each of these independent lines of evidence is found to be
significant in the cluster-outskirts; closer in, the data are more compatible
with uniform mixing. Our radial profile is qualitatively consistent with but
quantitatively tighter than previous results based on CN absorption. These
observations are qualitatively consistent with a scenario wherein a second
generation of stars with modestly enhanced helium and CNO abundance formed deep
within the gravitational potential of a cluster of previous generation stars
having more canonical abundances.

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    Astronomy and Astrophysics Review 01/2012; · 9.50 Impact Factor
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    ABSTRACT: Context. One of the greatest difficulties in astrophysics is the determination of the fundamental stellar parameters, one of which is the initial mass fraction of helium (Y). However, given that Y can be measured spectroscopically in only a small percentage of stars, a linear relationship is assumed between Y and the mass fraction of metals (Z) from a canonical perspective of the chemical evolution of the galaxies. This Y-Z relation is generally represented as Y = Yp + ΔY/ΔZ × Z, with the value of the helium-to-metal enrichment ratio (ΔY/ΔZ) assumed as a constant. However, there is no fundamental reason for every star to have a Y value on a linear scale with Z. Indeed, different ΔY/ΔZ values may be expected in different populations which have undergone different chemical enrichment histories. Aims: In this paper a new method for determining the fundamental stellar parameters of nearby stars is presented that uses at the same time Mbol, Teff, and log 1. One of these parameters is Y, which is used to determine the validity of the Y-Z relation. Methods: A new set of evolutionary tracks is created using the PGPUC stellar evolution code, which includes 7 masses (0.5 ≤ M/M⊙ ≤ 1.1), 7 helium abundances (0.230 ≤ Y ≤ 0.370), and 12 metallicities (1.6 × 10-4 ≤ Z ≤ 6.0 × 10-2) for solar-scaled chemical compositions ([α/Fe] = 0.0). The suggested method is tested using two different spectroscopic databases of nearby main sequence stars with precise parallaxes, and spectroscopic measurements of [Fe/H], Teff and 1. Results: The proposed method is compared to other techniques used to determine the fundamental stellar parameters, where one assumes an age of 5 Gyr for all nearby stars. This comparison demonstrates that the hypothesis regarding constant age leads to an underestimation of the Y value, especially for low metallicities. In addition, the suggested method is limited to masses above 0.60 M⊙ and requires high-precision measurements of spectroscopic surface gravities in order to obtain reliable results. Finally, estimating masses and Ages assuming a Y-Z relation rather than a free Y value may induce average errors of approximately 0.02 M⊙ and 2 Gyr, respectively.
    Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor


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